The present invention relates to the measurement of unidirectional current, and more particularly, to measurement of unidirectional output currents of multiphase power rectifiers.
Many circuits for measuring unidirectional current are known. Among such circuits are those disclosed in U.S. Pat. Nos. 3,281,642, issued Oct. 25, 1966 to Dortort, and 3,944,919, issued Mar. 16, 1976 to Jewell et al, the Jewell et al patent being assigned to the assignee of the present application.
Briefly, the Dortort patent discloses circuits for reducing measurement error due to a 30.degree. power circuit phase shift, as in a simple wye-delta or delta-wye three phase power circuit connection. In one such circuit, Dortort discloses delta connection of the current transformer secondaries associated with the primary line conductors and connection of these secondaries to a dc measuring instrument through a multiphase instrument rectifier. The Jewell et al patent discloses circuits for reducing the measurement error due to power circuit phase shift of 30.degree. as well as other magnitudes of phase shift. In one such circuit, Jewell et al provide current transformers in the input circuit of the power transformer with the current transformer secondary windings connected in wye circuit relation to a three phase instrument rectifier through a phase shifting current transformer. In one form of the Jewell et al circuit, the phase shifting transformer comprises an autotransformer.
Although the circuits disclosed in the Jewell et al patent are successful in many applications, for some applications, these circuits present some problems. One such problem is that the asymmetrical arrangement of the current autotransformers shown in FIGS. 6 and 7 of Jewell et al provide a non-unity magnitude transformation in addition to providing the desired phase transformation. This non-unity current magnitude transformation is undesirable as its presence requires additional current transformation circuitry in order to provide accurate measurement. More particularly, such additional current transformation circuitry must be provided in order that identical magnitude transformation is associated with groups of 6 pulse arrangements that are phase shifted with respect to each other. This allows the accurate summation (or comparison) and the determination of the total group current. Another problem with the circuits disclosed by Jewell et al is that, in some power systems, the power transformer primary circuit may involve a zero sequence current component. In such systems, the presence of the zero sequence component may be due to the distributed capacitance of a power transformer winding neutral to ground. In the Jewell et al circuit, no means are provided for handling the presence of the zero sequence current component. As a result, the presence of the zero sequence current may cause inaccurate measurement due to damage to the measurement circuitry. One means employed to provide zero sequence current capability is to connect the neutral connection of the wye connected current transformer secondaries to the ac terminal of a fourth bridge leg of the multiphase instrument rectifier. This provides zero sequence current protection but causes measurement error in the event that zero sequence current exists. Another problem with the Jewell et al circuit and with the Dortort circuit is that no means are provided for indicating the blowing of a fuse associated with the power rectifier circuit. This means that additional circuitry must be provided to indicate such a condition.
Thus, it is a general object of our invention to provide improved means for utilizing alternating input circuit current to measure dc output current in power rectifier apparatus without introducing error due to power transformer phase shift.
Another object of our invention is to provide such an improved measuring means which further includes means for rendering the current transformation substantially unity without the necessity of providing current transformation circuitry in addition to a phase shifting autotransformer.
Another object of our invention is to provide such an improved measuring means which includes means for handling the presence of zero sequence current without causing measurement error or circuit damage.
Another object of our invention is to provide such an improved measuring means which includes means for indicating a blowing of a fuse associated with the power rectifier circuit.